Wafer chuck having thermal plate with interleaved heating and cooling elements

ABSTRACT

A workpiece chuck includes a thermal plate assembly which includes both heating and cooling capability. The heating element can be a resistive heater in a coiled configuration disposed in a plane. The cooling can be performed via a cooling fluid circulated through cooling tubes which are also disposed in a coiled configuration in a plane. The plane of the heating element and the cooling tubes can be the same plane, and that plane can be a center plane of the thermal plate assembly. By locating the heating and cooling in the same plane, uniform heating and cooling are achieved. Also, by locating the heating element and cooling tubes in the center of the thermal plate, distortions such as doming and dishing in the thermal plate are eliminated such that the wafer can be held extremely flat on the chuck. The heating element and cooling tubes are coiled in an interleaved fashion to provide uniform heating and cooling while allowing them to simultaneously occupy the same plane.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/465,716, filed Jun. 19, 2003, which is a continuation of U.S. patentapplication Ser. No. 09/900,285, filed Jul. 6, 2001, now U.S. Pat. No.6,700,099, which is based on U.S. Provisional Patent Application Ser.No. 60/217,036, filed on Jul. 10, 2000. The contents of all of theseapplications and patent are incorporated herein in their entirety byreference.

BACKGROUND OF THE INVENTION

A workpiece chuck can be used to hold workpieces such as semiconductorwafers during processing and testing. Because integrated circuits formedin a wafer under test are commonly tested over temperature, theworkpiece chuck can include a temperature control system for controllingthe temperature of the wafer during testing. As integrated circuitsbecome smaller and more densely integrated, positioning tolerances fortesting systems such as wafer probers decrease. With very smallpositioning tolerances, it is very important that the chuck supportingthe wafer during testing be mechanically stable. This requirement isespecially challenging in a temperature-control chuck where the chuckmust maintain mechanical and electrical stability over a wide range oftemperatures. At temperature extremes, particularly at hightemperatures, conventional wafer chucks tend to distort due to thermalexpansion and contraction and the integrity of the materials of whichthe chucks are constructed. For example, chucks made of softer materialswill tend to distort at high temperature. This problem is exacerbated bythe increasing mechanical loads on the chuck introduced by the testsystem, i.e., the wafer prober.

SUMMARY OF THE INVENTION

The present invention is directed to various aspects of a workpiecechuck which overcome drawbacks of conventional chucks to provide a chuckwith improved mechanical and electrical stability. According to a firstaspect of the invention, there is provided a thermal control apparatus,or thermal plate assembly, and method, which can be used to controltemperature in a workpiece chuck. The thermal control apparatus includesa heating element and a cooling element. The heating element is disposedin a heating plane, and the cooling element is disposed in a coolingplane. The heating plane and the cooling plane can be coplanar, i.e.,they are the same plane.

The thermal plate assembly of the invention can be a layer in theworkpiece chuck. Because the heating and cooling elements occupy thesame horizontal plane of the chuck, the heating and cooling are uniformacross the top surface of the chuck where the workpiece, i.e., wafer, issupported. Also, because the heating and cooling elements are coplanar,substantial distortion and warping of the chuck and workpiece overtemperature are eliminated.

The heating element can include an electrical resistive heating coilelement. The heating coil element can be disposed in the heating planein a coiled configuration.

The cooling element can include one or more hollow tubes for circulatinga temperature-controlled fluid through the thermal plate assembly. Thecirculating tubes can be disposed in the cooling plane in a coiledconfiguration.

To facilitate locating both the heating element and the cooling elementin the same plane in coiled configurations, the heating and coolingelements are spatially interleaved with each other. The interleavednature of the heating and cooling elements also provides more uniformheating and cooling of the chuck and, therefore, more uniformtemperature across the surface of the wafer. Also, warping and otherdistortion of the chuck over temperature are substantially eliminated,such that the chuck can hold the wafer extremely flat over temperature.

In one embodiment, warping and other distortions over temperature arefurther reduced by the selection of the location of the heating andcooling plane within the thermal plate assembly. In this embodiment, theheating and cooling planes are located in a center plane of the thermalplate assembly, i.e., the plane that is equidistant from the top andbottom surfaces of the thermal plate assembly. With the heating andcooling planes located at the vertical center of the thermal plateassembly, distortions caused by doming and/or dishing of the thermalplate assembly are substantially eliminated. Again, with the reductionin chuck distortion over temperature, the wafer can be held flat overtemperature.

In one embodiment, the thermal plate assembly of the invention is madefrom a cast material, which, in one particular embodiment, is aluminum.The casting of the housing provides the thermal plate assembly withimproved mechanical rigidity and stability over temperature. The housingcasting can be stress relieved such as by heat treating at predeterminedmanufacturing steps. For example, stress relieving can be performed bothbefore and after finish machining of the housing. The stress reliefprovides the housing with more mechanical stability over temperature.Also, the housing casting can be formed with the tubes for circulatingthe cooling fluid. With the stress relief procedure, even moremechanical stability is provided.

In another aspect, the invention is directed to a workpiece chuckcapable of implementing interchangeable top surface assemblies. Inaccordance with this aspect, the workpiece chuck of the inventionincludes a lower support and the thermal plate assembly on the lowersupport. The top of the thermal plate assembly includes a mountapparatus capable of holding multiple types of top surface assemblies,which are used to hold the workpiece/wafer to the chuck.

This configuration provides the chuck of the invention with flexibilityaccording to the setting in which the chuck is being used. For example,one type of top surface assembly may be required where the test beingperformed requires the chuck to be able to absorb a large amount ofpower. In another test, the top surface assembly may be required toprovide low electrical capacitance, high voltage or high electricalisolation performance. In still another test, the top surface assemblymay be required to provide for very low signal leakage. In each of thesetests, the top surface assembly may be fabricated differently to provideoptimal performance under the specific testing parameters. In aconventional chuck system, this would require the user to obtain severaldifferent chucks, one for each test type. However, in accordance withthis aspect of the invention, the thermal plate assembly of theinvention provides a universal type mount which can accommodate all ofthe various top surface assemblies. As a result, the user need only havea single thermal plate assembly. The user can then purchase only the topsurface assemblies required for the tests to be performed. This resultsin considerable cost savings to the user.

In another aspect, the invention is directed to an approach toeliminating the negative effects resulting from the relative movement oflayers of the workpiece chuck over temperature. When two adjacent chucklayers have different thermal expansion coefficients, they tend to rubeach other over temperature. This can cause abrasion of the surfaceswhich can degrade chuck performance. This is especially true in oneparticular example where a top surface assembly made of a hard abrasiveceramic material is located adjacent to the cast aluminum housing of thethermal plate assembly. To reduce these effects, one or more adjacentsurfaces can be coated with a hard coating, such as hard anodize.

In accordance with this aspect, the invention includes a lower supportand the thermal plate assembly mounted on the lower support. An uppersupport, e.g., top surface assembly, by which the workpiece can bemounted to the chuck, is mounted over the thermal plate assembly. A hardcoating layer, a hard anodize layer for example, is adhered to a surfaceand interposed between the thermal plate assembly and the upper supportassembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 contains a schematic cross-sectional diagram of a thermal plateassembly in which the heating and cooling elements are not in the sameplane.

FIG. 2 contains a schematic cross-sectional diagram of a thermal plateassembly in which the heating and cooling elements are located in thesame plane at the center line of the thermal plate assembly, inaccordance with one embodiment of the invention.

FIG. 3 contains a schematic cross-sectional diagram of a thermal plateassembly in which the heating and cooling elements are in the same planelocated below the center line of the thermal plate assembly, toillustrate dishing on the top surface of the thermal plate assembly.

FIG. 4 contains a schematic cross-sectional diagram of a thermal plateassembly in which the heating and cooling elements are in the same planelocated above the center line of the thermal plate assembly, toillustrate doming on the top surface of the thermal plate assembly.

FIG. 5 contains a schematic top plan view of one embodiment of thethermal plate assembly of the invention, illustrating the interleavedconfiguration of the heating and cooling elements.

FIG. 6 contains a schematic cross-sectional view of one embodiment of aheater with multiple heating elements within a single sheath, inaccordance with the present invention.

FIG. 7 contains a schematic cross-sectional view of one embodiment of aworkpiece chuck including the thermal plate assembly of the invention.

FIGS. 8A through 8C contain schematic cross-sectional view of varioustypes of top surface assemblies, in accordance with the invention.

FIG. 9 contains a schematic exploded view of one embodiment of thethermal plate assembly of the invention with the mounting approach usedto accommodate multiple interchangeable types of top surface assemblies.

FIG. 10. contains a schematic partial cross-sectional view of a portionof the workpiece chuck according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The present invention is directed to and is applicable intemperature-controlled workpiece chucks of the type described in, forexample, U.S. Pat. No. 6,019,164, issued Feb. 1, 2000, entitled,“Workpiece Chuck,” assigned to Temptronic Corporation, and incorporatedherein in its entirety by reference; U.S. Pat. No. 6,073,681, issuedJun. 13, 2000, entitled, “Workpiece Chuck,” assigned to TemptronicCorporation, and incorporated herein in its entirety by reference; andcopending U.S. patent application Ser. No. 09/473,099, filed on Dec. 28,1999, entitled, “Workpiece Chuck,” assigned to Temptronic Corporation,and incorporated herein in its entirety by reference.

These chucks include a heater for heating the wafer under test and aheat sink for removing heat to cool the wafer. In accordance with thepresent invention, the heater and heat sink are formed in one integralassembly referred to herein as a thermal plate or thermal plateassembly. In one embodiment, the housing of the thermal plate assemblyis made of a cast material such as a metal. The cast metal providesmechanical strength, rigidity and stability over temperature. The castmaterial can be aluminum. It can be pure aluminum or an alloy ofaluminum or any material which provides low thermal distortion, i.e.,distortion over temperature due to temperature effects.

The heater includes one or more electrical resistive heating elementscoiled in a plane within the thermal plate cast housing. The heater caninclude multiple heating elements contained within a sheath. Because ofthe electrical current flowing through the heater elements duringheating, the heater elements are highly isolated from the remainder ofthe thermal plate assembly and the chuck to prevent interference withthe testing being performed. In one particular embodiment, the isolationof the heating elements is over 100 Gigohms. The heat sink portion ofthe thermal plate can include one or more tubes coiled in a plane withinthe thermal plate housing for circulating a temperature-controlled fluidthrough the plate.

The thermal plate is configured as a layer in the chuck. In general, thethermal plate is located between the base of the chuck by which thechuck is mounted on a host machine, e.g., a wafer prober, and the topsurface assembly on which the wafer under test is mounted. Heat from theheating elements is conducted through the plate casting up to the waferto heat the wafer. When cooling the wafer, heat is conducted downthrough the plate casting and is carried away by the circulating fluid.Thus, the combination of the heating elements and the circulating fluidallow the temperature of the chuck and the wafer it supports to be veryprecisely controlled. The temperature of the chuck and wafer can becontrolled via the thermal plate in accordance with copending U.S.patent application Ser. No. 09/001,887, filed on Dec. 31, 1997,entitled, “Temperature Control System for a Workpiece Chuck,” assignedto Temptronic Corporation, and incorporated herein in its entirety byreference.

In accordance with the invention, it is recognized that if the plane inwhich the heating elements are disposed and the plane in which the heatsink fluid tubes are disposed are offset from each other in the verticaldirection, then various problems result, including physical distortionsin the chuck. These distortion effects prevent the wafer from being heldflat during processing.

FIGS. 1-4 are schematic cross-sectional views of thermal platesillustrating possible layouts of the heating elements and cooing tubes.In FIG. 1, the general case in which the heaters and cooling tubes donot lie in the same plane is illustrated. Specifically, the case inwhich the heater 12 is located above the cooling tubes 14 within thecasting 10 is illustrated. In this case, because of thermal expansionand contraction effects, when the heater 12 and/or the cooling tubes 14are active, the housing 10 will tend to bow up in the middle, resultingin distortion in the chuck and the wafer.

FIG. 2 illustrates the configuration in accordance with the invention inwhich the heaters 12 and cooling tubes 14 are disposed in the sameplane. In this configuration, the distortions due to the heaters 12 andtubes 14 being in different planes are eliminated.

In accordance with the invention, it is also recognized that it ispreferable that the plane in which the heaters 12 and tubes 14 aredisposed be located along the center of the housing 10 in the verticaldimension. That is, the plane in which they are disposed should includethe horizontal center line 16 shown in FIG. 2. It is recognized that ifthe heaters 12 and tubes 14 are not located at the center of the thermalplate 10, then distortions result as shown in FIGS. 3 and 4. FIG. 3illustrates the situation in which the heaters 12 and tubes 14 arelocated below the casting center line 16. In this case, the cast housing10 bows down when heat is applied, resulting in a dish shape at the topsurface of the chuck and the wafer. When cooling is applied, theopposite occurs; i.e., the housing 10 bows up, resulting in a dome shapeat the top surface of the chuck and wafer. FIG. 4 illustrates thesituation in which the heaters 12 and tubes 14 are located above thecasting center line 16. In this case, the cast housing 10 bows up whenheat is applied, resulting in a dome shape at the top surface of thechuck and the wafer. When cooling is applied, the housing 10 bows down,resulting in a dish shape at the top surface of the chuck and the wafer.Hence, it is preferred that the plane in which the heaters 12 and tubes14 are located be at the center line of the cast housing 10.

FIG. 5 is a schematic top plan view of the thermal plate 10 of theinvention with heating elements 12 and coolant circulating tube 14.Because the heaters 12 and cooling tube 14 are coiled in the same plane,they are interleaved with each other as shown. The heater 12 iselectrically connected to a power source (not shown) via a connector 18and wires 20. As described below in connection with FIG. 6, the heater12 can actually include multiple heating elements enclosed in a sheath22. The cooling fluid is circulated through the thermal plate 10 by tube14. The fluid enters the plate 10 at an inlet port 24, flows in a spiralfashion through the coiled tube 14 to the center of the plate 10 andflows back out to the edge of the plate 10 in a spiral fashion in thecoiled tube 14. The fluid exits the plate 10 at an outlet port 26. Boththe heater 12 and cooling tube 14 are coiled in a spiral fashion withinthe plate 10 to provide efficient and uniform heat transfer to and fromthe chuck and wafer. This ensures highly accurate and uniformtemperature setting and wafer testing.

FIG. 6 is a schematic cross-sectional view of one embodiment of a heater12 in accordance with the invention. The heater 12 actually includesmultiple, four in this case, electrical resistive heating elements 40which are enclosed in a rigid or flexible and formable sheath enclosure42. In the system described herein, the sheath 42 is formable such thatthe coiled configuration can be obtained. The heating elements 40 aresupported and insulated from each other within the sheath 42 by aninsulating and supporting material 44. The insulating and supportingmaterial 44 achieves extremely high electrical isolation of the heatingelements 40 from each other, the sheath, which is electrically connectedto the thermal plate assembly, and the rest of the chuck. In oneembodiment, the insulating and supporting material 44 is made fromhighly compressed insulating material such as magnesium oxide. Becauseit is highly compressed, moisture is prevented from contaminating thematerial and reducing its isolation characteristics. In one embodiment,the heater assembly 12 achieves over 100 Gigohms of isolation betweenthe conductive elements and the sheath 42. The heater 12 includeshermetically sealed ends 48 and insulated heater leads 46 at one or bothends for attachment to a power source (not shown). Where the powersource is connected at only one end, leads at the opposite end of theheater 12 can be connected together. In FIG. 6, these connections 47 areshown in phantom at the right end of the heater 12. In this case, theleft set of leads 46 can be connected to the power source to create twoheater elements.

In accordance with the invention, steps are taken during fabrication ofthe thermal plate cast housing to ensure that distortion in the thermalplate and chuck due to thermal effects are eliminated and that theentire system exhibits superior thermal and mechanical performance. Forexample, the aluminum casting is heat treated for stress relief aftercasting and before finish machining operations are performed. Thecasting is heat treated for stress relief again after finish machiningsuch that distortion in the finished casting due to thermal effects iseliminated. In one embodiment, the heating and cooling elements are castinto the aluminum housing, which be cylindrical or non-circular. Theheating and cooling coils are also stress relieved.

Referring to FIG. 7, in one configuration, the thermal plate 10 servesas a layer of the wafer chuck 50. The thermal plate 10 is mounted over achuck base 11 by which the chuck 50 is mounted on the host machine. Thetop surface assembly 52 of the chuck 50 is mounted on the top surface ofthe thermal plate 10. The top surface assembly 52 supports and holds thewafer 5 for processing. The top surface assembly 52 is held on the topof the thermal plate 10 by vacuum, and the wafer 5 is held on the topsurface assembly 52 by vacuum. To that end, vacuum input ports 15 and 17are provided in the thermal plate 10. Vacuum input port 15 is coupled tovacuum channels which convey vacuum to the top of the thermal plate 10.A vacuum pattern including vacuum grooves or channels is formed in thetop of the thermal plate 10 to distribute vacuum over the top of thethermal plate to hold the top surface assembly 52 to the thermal plate10. Vacuum input port 17 is coupled to vacuum channels which conveyvacuum up through the top surface assembly 52 to its top surface. Avacuum pattern including vacuum grooves or channels is formed in the topof the surface assembly 52 to distribute vacuum over the top of thesurface assembly to hold the wafer 5 to the top surface assembly 52.

In one embodiment, the configuration of the top surface assembly 52 canbe selected based upon the type of test being performed on the wafer 5.Different surface assemblies are used for different tests. In accordancewith the invention, different top surface assemblies 52 can beinterchanged according to the test being performed. The thermal plate 10is configured to accommodate each type of top surface assembly 52, andthe surface assemblies 52 are all made to be mounted on the thermalplate 10. Thus, the thermal plate 10 serves as a temperature-controlledvacuum mount platform having a unique adaptability to multiple testingperformance requirements.

FIGS. 8A through 8C illustrate three different types of top surfaceassemblies 52A through 52C, respectively, which can be attached to thetop of the thermal plate 10 for different tests. For example, one typeof test being performed may require that high device power be absorbedby the surface 52, which would require that the surface 52 include avery thermally conductive material. In such a test, the top surfaceassembly 52A in FIG. 8A can be used. Surface assembly 52A includes alayer 60 of conductive material such as copper or aluminum covered by aplated or sputtered layer 62 of a conductive material such as gold,nickel or other such material. Another test may have very low electricalcapacitance, high voltage or high isolation requirements. In such acase, the surface 52B of FIG. 8B can be used. Surface 52B includes aninsulating dielectric layer 64, typically a ceramic material, covered bya plated or sputtered layer 62 of a conductive material such as gold,nickel or other such material. In another testing application, there maybe a need for very low electrical signal leakage. This would require thesurface assembly 52C of FIG. 8C. Surface assembly 52C includes a lowerinsulator layer 70 on which is mounted a conductive guard layer 68. Anupper insulator layer 66 is positioned over the guard layer 68, and aplated or sputtered layer 62 of a conductive material such as gold,nickel or other such material is formed on the upper insulator 66.Alternatively, the top conductive layer 62 can be held by vacuum asdescribed below in connection with FIG. 9. The guard layer 68 is drivenwith an excitation signal during testing to minimize test signal leakagedue to isolation effects.

Each of the surfaces 52A through 52C is made to be of the same totalthickness X such that they do not introduce a height difference whenthey are mounted on the thermal plate 10. This is true regardless of thenumber of layers in the assembly 52. Locating pins are provided betweenthe thermal plate 10 and the surface assembly 52 as an aid in mountingthe surface 52 on the plate 10. The locating pins also serve to hold thecomponents together when vacuum is not present. Mechanical latches canalso be used to secure the components when vacuum is removed.

As noted above, vacuum is used to hold the top surface assembly 52 tothe thermal plate 10. Vacuum is also used to hold individual layers ofthe surface assembly 52 together. For example, FIG. 9 is a schematicexploded view of the surface 52C of FIG. 8C. As shown in FIG. 9, the topof the thermal plate includes a vacuum pattern 71 used to hold the lowerinsulator layer 70. The reference “V” refers to the vacuum grooves. Thelower insulator 70 includes a vacuum pattern 73 used to hold the guardlayer 68. Vacuum passes through vacuum holes in the guard layer 68 up toand through the upper insulator layer 66 where a vacuum pattern 75 onboth sides of layer 66 is used both to hold a top layer 62 and to holdlayer 66 to layer 68. FIG. 9 also illustrates the locating pins 7 usedto hold the layers in position.

FIG. 10 is a schematic partial cross-section of the thermal plate 10with the surface assembly 52C of FIG. 9 mounted thereon. FIG. 10illustrates the vacuum ports 15 and 17 and distribution of the vacuum tothe layers of the surface assembly 52C.

The thermal plate 10 and other components of the chuck of the inventioncan also include a surface finish that has both high durability andelectrical isolation characteristics. In applications where the thermalplate and the top surface assembly 52 have different thermal expansioncoefficients, the two components will move relative to each other astemperature is changed. As a result, abrasion occurs at the interfacebetween the two components. In one application, the surface 52 is a veryhard ceramic material and the thermal plate is much softer material suchas cast aluminum. The softer thermal plate 10 is scratched and abraded.This is especially true at high temperatures, where the ceramic andaluminum tend to bond to each other. In accordance with the invention, ahard surface covering, such as a hard anodize plating, is provided onthe top surface of the thermal plate 10 to eliminate the abrasion andbonding between the thermal plate 10 and the surface assembly 52.

In another application, the bottom of the surface assembly 52 is metal.The hard anodize coating can be applied to either the top surface of thethermal plate 10 or the bottom surface of the surface assembly 52. Inthis case, the anodize coating provides electrical isolation between thethermal plate 10 and the surface assembly.

In still another application, the hard anodize coating is applied to thetop of the thermal plate 10. Then, a conductive layer is bonded to theanodized layer. Next, a final layer of anodize is applied to the top ofthe conductive layer. The conductive top surface layer 62 of the surfaceassembly 52 is then mounted on the final anodize layer. In thisconfiguration, the conductive layer between the two anodize layers canbe used as a guard layer. The electrically conductive guard plate isinsulated by the layers of anodize from both the thermal plate 10 andthe surface assembly 52. Therefore, thisanodize-conductor-anodize-conductor structure can be used instead of thetop surface assembly 52C of FIG. 8C. This approach is considerably lessexpensive and simpler to implement than the multilayer structure of FIG.8C. Also, the guard layer can be made inexpensively and quickly byanodizing a thin sheet of conductive material such as aluminum on bothsides or all over its exterior.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the following claims.

1. A thermal control apparatus for a workpiece chuck, the thermalcontrol apparatus comprising: a heating element disposed in a heatingplane; and a cooling element disposed in a cooling plane; wherein theheating plane and the cooling plane are the same plane.
 2. The thermalcontrol apparatus of claim 1, wherein the heating element comprises aresistive heating element in a coil configuration within the heatingplane.
 3. The thermal control apparatus of claim 1, wherein the coolingelement comprises at least one tube for circulating cooling fluid, thetube being configured in a coil configuration in the cooling plane. 4.The thermal control apparatus of claim 3, wherein the heating elementcomprises a resistive heating element in a coil configuration within theheating plane.
 5. The thermal control apparatus of claim 4, wherein theheating element is electrically isolated from the thermal plateassembly.
 6. The thermal control apparatus of claim 5, wherein theisolation impedance is over 100 Gigohms.
 7. The thermal controlapparatus of claim 4, wherein the resistive heating element and the tubeare disposed in a plane in coil configurations interleaved with eachother.
 8. The thermal control apparatus of claim 4, wherein the plane inwhich the resistive heating element and the tube are disposed is acenter plane of the thermal control apparatus halfway between a topsurface of the thermal control apparatus and a bottom surface of thethermal control apparatus.
 9. The thermal control apparatus of claim 1,wherein the heating plane and the cooling plane lie in a center plane ofthe thermal control apparatus halfway between a top surface of thethermal control apparatus and a bottom surface of the thermal controlapparatus.
 10. The thermal control apparatus of claim 1, furthercomprising a housing enclosing the heating element and the coolingelement, the housing being made from a cast metal.
 11. The thermalcontrol apparatus of claim 10, wherein the metal comprises aluminum. 12.The thermal control apparatus of claim 10, wherein the metal is purealuminum.
 13. The thermal control apparatus of claim 10, wherein themetal is an alloy with low thermal distortion.
 14. The thermal controlapparatus of claim 1, wherein the thermal control apparatus is a layerof the workpiece chuck.
 15. The thermal control apparatus of claim 14,wherein the heating element comprises a resistive heating element in acoil configuration within the heating plane.
 16. The thermal controlapparatus of claim 14, wherein the cooling element comprises at leastone tube for circulating cooling fluid, the tube being configured in acoil configuration in the cooling plane.
 17. The thermal controlapparatus of claim 16, wherein the heating element comprises a resistiveheating element in a coil configuration within the heating plane. 18.The thermal control apparatus of claim 17, wherein the resistive heatingelement and the tube are disposed in a plane in coil configurationsinterleaved with each other.
 19. The thermal control apparatus of claim17, wherein the plane in which the resistive heating element and thetube are disposed is a center plane of the thermal control apparatushalfway between a top surface of the thermal control apparatus and abottom surface of the thermal control apparatus.
 20. The thermal controlapparatus of claim 14, wherein the heating plane and the cooling planelie in a center plane of the thermal control apparatus halfway between atop surface of the thermal control apparatus and a bottom surface of thethermal control apparatus.
 21. The thermal control apparatus of claim14, further comprising a housing enclosing the heating element and thecooling element, the housing being made from a cast metal.
 22. Thethermal control apparatus of claim 21, wherein the metal comprisesaluminum.